This invention relates to measuring instruments such as industrial recorders which measure the voltage generated by primary elements in response to a physical quantity. More particularly, this invention relates to an improvement in the preamplifier section of such an instrument and specifically to those adjustments in the preamplifier section dedicated to the adjustment of the span and zero of the instrument for calibration purposes.
For the purposes of this description, the span of an instrument such as a voltage recorder will be considered as that range of values for the measured variable covered by a full scale travel of the instrument printer.
The zero of an instrument shall be considered as that point on the scale or fictitious point corresponding to a value for the measured variable of zero volts.
The prior art devices have usually utilized means for making the span and zero adjustments which were incorporated in the preamplifier circuit in such a way that they produced an interaction, one upon the other. Thus, with the prior art instruments, whenever it was necessary to calibrate the instrument by adjusting either the span adjustment or the zero adjustment, it was also necessary to adjust the other one and then refine the original adjustment, thus carrying out the interaction necessary in order to take care of the interacting nature of those adjustments.